Francis Turbine || Operation and working principle

As we know, turbine is an important equipment in power plant which is used in electrical power generation. We have already discussed about turbine, turbine types. Here we are going to discuss about Francis Turbine || Operation and working principle. As we know, Francis turbine is reaction turbine where its pressure will be different in inflow and outflow through out the turbine. Due to pressure change, it create rotational motion on turbine for rotation which is concept of reaction. Turbine shaft is coupled with the shaft of generator or connected to generator shaft by different mechanism like belt system for electricity production. A simple picture of Francis turbine is given below.

Francis Turbine

This is a mixed flow reaction turbine which is used for low head and high discharge. It is invented by James Bicheno Francis ( 1815 - 1892) who is a civil engineer. Water is flow from certain height (called head race) to tail race through penstock, water will flow on turbine in that process turbine blades will also move/rotate with water flow. So that it is usually used in low to medium head and medium to high flow rate. Here, pressure will be changed as compared to turbine inlet and turbine outlet and on that basis turbine runner will rotate.

WORKING PRINCIPLE OF Francis turbine

In this, water under pressure enters the runner from the guide vanes is radial direction and discharge out of the runner axially. When water flow reach its inlet, discharge with high pressure will move towards the center of turbine and water flow will rotate on it. On that process, runner will also rotate with water rotation. There is guide vane which will guide the discharge while rotation and scroll case will provide way to flow discharge from penstock to turbine and also help to let not water go outside or splashed water will go on tail race so that it will not harm generator, turbine and power house place.

Water stored in dam/reservoir is move to turbine through penstock. While water discharge enter to runner, it has low velocity because of low head but with high discharge which creates high pressure on it. This high pressure will converted into velocity head and hit the blades/vane & move on runner blades so that runner will also move with discharge. In this way, runner will run with water flow/discharge.

Components/parts of Francis Turbine

1. Casing :- It is also known as spiral casing or scroll casing. It is used to convey water from penstock to turbine so that runner will rotate smoothly. 

2. Guide vane or Guide mechanism :- This is a stationary circular wheel on runner so that water will strike or hit these fixed vanes. Due to this strike, runner will rotate with water flow without shock at inlet. Their surface is made in such a way that when water enter and leaves the runner, it will not create shock on turbine. These guide vanes are fixed on guide mechanism and their surface is made very smooth.

3. Runner :- This is a circular wheel of turbine which will rotate by flow. A number of radial curved vanes are attached on it so that when water inlet through casing and hit these vanes, runner will rotate. The number of curved guide vanes, varies from 12 to 22, are attached on runner which is depended on specific speed.

4. Draft tube :- This is a tube or pipe whose diameter is gradually increasing and connected to outlet of runner to tail race. This is used to convey the water to tailrace from runner for that its one end is connected to outlet of runner and another end is connected to tail race which is submerged below the water's level. This is connected to turbine because it helps to increase the efficiency of the turbine and also help to recover the velocity head of water i.e. to increase the net head so that helps to produce more power generation.

Here is an animation figure of Francis turbine.